ESD protection for bipolar-CMOS-DMOS integrated circuit devices

ABSTRACT

An Electro-Static Discharge (ESD) protection device is formed in an isolated region of a semiconductor substrate. The ESD protection device may be in the form of a MOS or bipolar transistor or a diode. The isolation structure may include a deep implanted floor layer and one or more implanted wells that laterally surround the isolated region. The isolation structure and ESD protection devices are fabricated using a modular process that includes virtually no thermal processing. Since the ESD device is isolated, two or more ESD devices may be electrically “stacked” on one another such that the trigger voltages of the devices are added together to achieve a higher effective trigger voltage.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 11/499,381,filed Aug. 4, 2006, which is incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

This invention relates to semiconductor chip fabrication and inparticular to methods of fabricating and electrically isolating bipolar,CMOS and DMOS transistors and passive components in a semiconductor chipmonolithically at high densities without the need for high temperaturefabrication processing steps, and to provide ESD protection for suchdevices.

BACKGROUND OF THE INVENTION

In the fabrication of integrated circuit (IC) chips, it is frequentlynecessary to electrically isolate devices that are formed on the surfaceof the chip, especially when these components operate at differentvoltages. Such complete electrical isolation is necessary to integratecertain types of transistors including bipolar junction transistors andvarious metal-oxide-semiconductor (MOS) transistors including power DMOStransistors. Complete isolation is also needed to allow CMOS controlcircuitry to float to potentials well above the substrate potentialduring operation. Moreover, complete isolation allows the design ofnovel Electro-Static Discharge (ESD) protection devices.

Ability to survive an ESD event is one of the key requirements for ICs.A common method for providing such ESD protection is to include one ormore ESD clamp devices that are connected across the external pins of anIC. More generally, the ESD devices are connected between the inputterminals of, and thus in parallel with, the circuitry that is to beprotected. These clamp devices are generally designed to break down at avoltage below that which would cause damage to the internal circuitry ofthe IC, thus absorbing the ESD energy and protecting the IC circuitry.The most commonly used ESD clamp devices are simple diodes, NPN bipolartransistors, and grounded-gate NMOS (GGNMOS) transistors, which aredesigned to operate in the bipolar snapback mode.

FIGS. 1A-1C show two prior art ESD clamp devices. GGNMOS device 100 inFIG. 1A comprises a NMOS transistor 101 with drain (D) connected toinput pad 102 and source (S) connected to ground pad 103. The NMOS gate(G) is connected to source through a gate resistor 104, with a valuetypically in the range of 1 kohm-100 kohm, and the NMOS body (B) isconnected to source through internal body resistance 105 that isoptimized to allow the GGNMOS to snapback due to parasitic NPN bipolaraction at a reasonably low drain voltage. NPN ESD clamp device 110 inFIG. 1B comprises an NPN transistor 111 with collector (C) connected toinput pad 112 and emitter (E) connected to ground pad 113. The NPN base(B) is connected to emitter through internal base resistance 114 toallow the NPN to snapback due to BVcer at a reasonably low collectorvoltage.

FIG. 2 shows a cross-section schematic of prior art GGNMOS device 100from FIG. 1A. In this conventional, non-isolated CMOS process, P-wellregion 201, which serves as the body of the NMOS, is formed in P-typesubstrate 202. Therefore the body of this prior art GGNMOS is alwaysconnected to the substrate potential (“ground”). The device alsoincludes N+ drain region 203, N+ source regions 204A and 204B, P+contact region 205, lightly-doped drain (LDD) regions 206, gate 207,gate oxide 208, sidewall spacers 209, field oxide 210, inter-leveldielectric (ILD) 211, and metal layer 212.

FIG. 3 shows a cross-section schematic of prior art NPN ESD clamp device110 from FIG. 1B. In this conventional, non-isolated CMOS process,P-well region 301, which serves as the base of the NPN, is formed inP-type substrate 302. Therefore the body of this prior art NPN ESD clampis always connected to the substrate potential (“ground”). The devicealso includes N+ collector region 303, N+ emitter regions 304A and 304B,P+ contact region 305, field oxide 310, ILD 311, and metal layer 312.

The breakdown or trigger voltage of ESD clamp devices is typicallylimited to less than 20V by the vertical breakdown of various junctionsin a given process. ESD devices with higher trigger voltages generallyrely on a lateral breakdown mechanism that is prone to current crowding,making it difficult to design large structures that effectivelydistribute the ESD energy. The use of series connected or “stacked” ESDclamp devices would allow the trigger voltages of a several ESD clampdevices to be added to achieve higher total trigger voltage, but thisrequires complete isolation of the ESD clamp devices.

Fabrication of conventional CMOS in P-type substrate material does notfacilitate complete isolation of its devices since every P-type wellforming the body (back-gate) of NMOS transistors is shorted to thesubstrate potential, typically the most negative on-chip potential. Onemethod for achieving complete isolation is epitaxial junction-isolation,which employs an N-type epitaxial layer grown atop a P-type siliconsubstrate and separated into electrically isolated tubs by a deep P-typeisolation diffusion—one requiring high temperature processes toimplement. High temperature processing causes a redistribution of dopantatoms in the substrate and epitaxial layers, causing unwanted tradeoffsand compromises in the manufacturing of dissimilar devices fabricatedusing one common process. Moreover, the high-temperature diffusions andepitaxy employed in epi-JI processes are generally incompatible with thelarge wafer diameters and advanced low-temperature processing equipmentcommon in submicron CMOS fabs.

What is needed is a process for integrating various IC devices with ESDprotection devices that allows for the formation of stacked devices, yeteliminates the need for high temperature processing and epitaxy.Ideally, such a manufacturing process should employ “as-implanted”dopant profiles—ones where the final dopant profiles remainsubstantially unaltered from their original implanted profiles by anysubsequent wafer processing steps. Moreover, the process should beconstructed in a modular architecture where devices may be added oromitted and the corresponding process steps added or removed to theintegrated flow without changing the other devices available in theprocess's device arsenal.

SUMMARY OF THE INVENTION

The clamping devices of this invention are formed within an isolatedregion of a substrate of a first conductivity type. The isolated regionis bounded on the bottom by a deep implanted floor layer of a secondconductivity type opposite to the first conductivity type and on thesides by one or more implanted wells of the second conductivity typethat extend downward from the surface of the semiconductor material andmerge with the deep implanted layer. In many embodiments the isolatedregion is bounded on the side by a single well that is formed in theshape of a closed figure—for example, a circle, rectangle or otherpolygon or some other shape.

A variety of ESD protection devices may be formed within the isolatedregion. For example, in one embodiment a bipolar transistor is formed inthe isolated region, with its base connected to its emitter through aresistance such that a two-terminal device is formed. In anotherembodiment, a grounded-gate MOS device is formed with both its bodyregion and its gate connected to its drain through respectiveresistances.

In yet another group of embodiments, a clamping diode is formed in theisolated region. The isolated device is formed in a P-type substrate andthe floor isolation layer and the well(s) that surround the isolatedregion laterally are N-type. An N+ cathode region is formed at thesurface of the isolated region and a P anode region is formed beneaththe N+ cathode region. The P anode region may be formed by a successionof chained implants with the deeper implants having a higher dopingconcentration than the shallower implants. Alternatively, the anode andcathode may be formed by a series of parallel N-type and P-type regionswithin the isolated region.

The doped regions that constitute the isolation structure and the dopedregions that constitute the ESD protection device are preferably formedby single or multiple implants with essentially no thermal processesthat would result in the diffusion of the dopants. These doped regionstherefore remain in an essentially “as-implanted” configuration. Theprocess flow is modular in the sense that, with a few exceptions, theimplants may be performed in virtually any order, and it is possible toeliminate one or more process steps in the fabrication of a given IC,depending on which set of devices are required.

The ESD protection are connected between the input terminals of thecircuitry that is to be protected. Since the ESD protection devices areisolated from the substrate, they can be series connected or “stacked”such that the trigger voltages of a several ESD clamp devices are addedtogether to achieve a higher effective trigger voltage in order toprovide protection for high voltage circuits.

DESCRIPTION OF FIGURES

FIGS. 1A-1C are schematic circuit diagrams of prior art ESD protectiondevices.

FIG. 2 is a cross-sectional view of a prior art GGNMOS ESD clamp device.

FIG. 3 is a cross-sectional view of a prior art NPN ESD clamp device.

FIGS. 4A-4D are schematic circuit diagrams of stacked ESD protectiondevices.

FIG. 5 is a cross-sectional view of an isolated NPN ESD clamp device.

FIG. 6 is a cross-sectional view of an isolated GGNMOS ESD clamp device.

FIGS. 7A-7C are cross-sectional views of an isolated ESD clamp diodes.

FIG. 8 is a cross-sectional view of, a stacked ESD clamp structure.

DESCRIPTION OF THE INVENTION

An all low-temperature fabrication method using as-implanted junctionisolation structures employs high-energy and chain implants with dopantimplanted through contoured oxides to achieve fully-isolated bipolar,CMOS and DMOS devices without the need for isolation diffusions, epitaxyor high temperature processes. The low-temperature wafer fabricationmethods and isolated device structures were previously described inpending U.S. Application No. 11/298,075 and in U.S. Pat. Nos. 6,855,985,6,900,091 and 6,943,426 to R. K. Williams et al., each of which isincorporated herein by reference.

The inventive matter in this application is related to these patents andapplications but concentrates on the design and integration of isolatedand stackable ESD protection structures.

The low-temperature fabrication of the high-voltage devices described inthis application are compatible with the modular low-temperaturefabrication methods described in the aforementioned applications, butare not necessarily limited to modular process architectures.

While specific embodiments of this invention have been described, itshould be understood that these embodiments are illustrative only andnot limiting. Many additional or alternative embodiments in accordancewith the broad principles of this invention will be apparent to those ofskill in the art.

Wafer Fabrication

Except as specifically stated, wafer fabrication of the devicesdescribed herein utilizes the same process sequence that is described inthe above referenced patents. A brief summary of the basic process flowincludes

Field oxide formation

High-energy implanted deep drift layer (ND) formation

High-energy implanted floor isolation (DN) formation

1^(st) chain-implanted non-Gaussian N-well (NW1/NW1B) formation

1^(st) chain-implanted non-Gaussian P-well (PW1/PW1B) formation

2^(nd) chain-implanted non-Gaussian N-well (NW2/NW2B) formation

2^(nd) chain-implanted non-Gaussian P-well (PW2/PW2B) formation

Dual gate oxide and gate electrode formation

N-base implant

P-base implant

1^(st) N-LDD implant (NLDD1)

1^(st) P-LDD implant (PLDD1)

2^(nd) N-LDD implant (NLDD2)

2^(nd) P-LDD implant (PLDD2)

ESD implant

Sidewall spacer formation

N+ implant

P+ implant

Rapid thermal anneal (RTA) implant activation

Multilayer metal interconnect process

Passivation

Since the process as described utilizes “as-implanted” dopant profileswith little or no dopant redistribution, implants may be performed invirtually any order except that it is preferred that the P-well andN-well implantation precede gate formation, the trench gate formationprecede DMOS body implantation, N-LDD and P-LDD implants follow gateformation but precede sidewall spacer formation, and N+ and P+ implantsfollow sidewall spacer formation. This process flow is designed to bemodular, so it is possible to eliminate one or more process steps in thefabrication of a given IC, depending on which set of devices arerequired for that IC design.

By way of example, the table below summarizes a preferred embodiment anda preferred range of conditions for the implants described in thisapplication:

Implant Preferred Embodiment (Species) (Energy, Dose) Preferred Range(Energy, Dose) DN (P⁺) E = 2.0 MeV, Q = 2E13 cm⁻² E = 1.0 MeV to 3.0keV, Q = 1E12 to 1E14 cm⁻² ND deep drift E = 800 keV, Q = 2E12 cm⁻² E =400 keV to 1.2 MeV, Q = 5E11 to 5E12 cm⁻² (P⁺) E = 600 keV, Q = 2E12cm⁻² E = 300 keV to 900 keV, Q = 5E11 to 5E12 cm⁻² P-body (B⁺) E = 120keV, Q = 2E12 cm⁻² E = 60 keV to 180 keV, Q = 5E11 to 5E12 cm⁻² E = 80keV, Q = 4E12 cm⁻² E = 40 keV to 120 keV, Q = 1E12 to 1E13 cm⁻² 1stP-well + E = 240 keV, Q = 1E13 cm⁻² E = 120 keV to 360 keV, Q = 5E12 to5E13 cm⁻² (B⁺) E = 120 keV, Q = 6E12 cm⁻² E = 60 keV to 180 keV, Q =1E12 to 1E13 cm⁻² 1st N-well + E = 460 keV, Q = 5E12 cm⁻² E = 230 keV to690 keV, Q = 1E12 to 1E13 cm⁻² (P⁺) E = 160 keV, Q = 1E12 cm⁻² E = 80keV to 240 keV, Q = 5E11 to 5E12 cm⁻² 2nd P-well + E = 460 keV, Q = 1E13cm⁻² E = 230 keV to 690 keV, Q = 5E12 to 5E13 cm⁻² (B⁺) E = 160 keV, Q =1E12 cm⁻² E = 80 keV to 240 keV, Q = 5E11 to 5E12 cm⁻² 2nd N-well + E =950 keV, Q = 1E13 cm⁻² E = 500 keV to 1.5 MeV, Q = 5E12 to 5E13 cm⁻²(P⁺) E = 260 keV, Q = 1E12 cm⁻² E = 130 keV to 390 keV, Q = 5E11 to 5E12cm⁻² N-base (P⁺) E = 300 keV, Q = 2E12 cm⁻² E = 150 keV to 450 keV, Q =5E11 to 5E12 cm⁻² E = 120 keV, Q = 9E12 cm⁻² E = 60 keV to 180 keV, Q =5E12 to 5E13 cm⁻² P-base (B⁺) E = 240 keV, Q = 6E12 cm⁻² E = 120 keV to360 keV, Q = 1E12 to 1E13 cm⁻² E = 100 keV, Q = 6E12 cm⁻² E = 50 keV to150 keV, Q = 1E12 to 1E13 cm⁻² NLDD1 (P⁺) E = 80 keV, Q = 2E13 cm⁻² E =40 keV to 160 keV, Q = 5E12 to 5E13 cm⁻² PLDD1 (BF₂ ⁺) E = 80 keV, Q =2E12 cm⁻² E = 40 keV to 160 keV, Q = 5E11 to 5E12 cm⁻² NLDD2 (P⁺) E = 80keV, Q = 6E12 cm⁻² E = 40 keV to 160 keV, Q = 1E12 to 1E13 cm⁻² PLDD2(BF₂ ⁺) E = 100 keV, Q = 3E12 cm⁻² E = 50 keV to 150 keV, Q = 1E12 to1E13 cm⁻² NESD (P⁺) E = 40 keV, Q = 1E15 cm⁻² E = 20 keV to 150 keV, Q =1E14 to 5E15 cm⁻² N+ (As⁺) E = 30 keV, Q = 5E15 cm⁻² E = 20 keV to 60keV, Q = 1E15 to 1E16 cm⁻² P+ (BF₂ ⁺) E = 30 keV, Q = 3E15 cm⁻² E = 20keV to 60 keV, Q = 1E15 to 1E16 cm⁻²

Using this process architecture, a number of unique ESD protectiondevices may be fabricated and integrated into an IC in a modularfashion. These new ESD devices include isolated diodes, GGNMOS, and NPNdevices. An important feature of these devices is the complete isolationprovided by a high-energy implanted floor isolation layer (DN). Sincethese devices are isolated from the substrate, they can be seriesconnected or “stacked” such that the trigger voltages of a several ESDclamp devices are added together to achieve a higher effective triggervoltage in order to provide protection for high voltage circuits.Stacking two devices that each have a 16V trigger voltage, for example,yields a combined trigger voltage of 32V, which may be suitable forprotection of 30V circuitry. Formation of such stacked devices is simplynot possible using prior art non-isolated CMOS processes, and while itis theoretically possible using epitaxial junction isolation techniques,the size of the ESD clamps would be prohibitive. Thus, the ESD devicesof this invention are unique in their combination of isolation andcost-effectiveness.

FIGS. 4A-4D show a circuit schematic of stacked ESD clamp structures400A-400D, respectively, each comprising a top ESD clamp 401 and abottom ESD clamp 402 connected in series between an input pad 403 and aground pad 404 and in parallel with a circuit 410 that is to beprotected. Bottom ESD clamp 402 may be non-isolated (having a commonterminal connected to the substrate) or isolated from the substrate. Itmay comprise any of several possible ESD clamp devices, including aGGNMOS as shown in FIG. 4A, an NPN ESD clamp as shown in FIGS. 4B and4D, an ESD clamp diode as shown in FIG. 4C, or other related devices.Top ESD clamp 401 is isolated from the substrate such that it can floatto a high voltage and thus be stacked in series with bottom ESD clamp402. Top ESD clamp 401 may comprise any of several possible ESD clampdevices, including a GGNMOS, as shown in FIG. 4A, an NPN ESD clamp asshown in FIG. 4B, an ESD clamp diode as shown in FIGS. 4C and 4D, orother related devices. The top and bottom ESD clamp devices may the sametype, or different types of devices may be used for the top and bottomclamps, respectively. For example, an NPN ESD clamp may be used on thebottom and an isolated ESD clamp diode on the top, as shown in stackedESD clamp structure 400D in FIG. 4D.

FIG. 5 shows a cross-sectional view of an isolated NPN ESD clamp 500. AnN+ collector 503 is separated from N+ emitters 504A and 504B by asignificant distance, for example 10-100 um, to provide some ballastingresistance to distribute the ESD current uniformly. P+ base contacts505A and 505B are spaced from the emitters 504A and 504B by asignificant distance, for example 10-100 um, to provide some resistancebetween the base 501 and emitters 504A and 504B, which lowers thebipolar snapback voltage and allows easier triggering during an ESDevent. An optional ESD implant 516 may be included adjacent thecollector 503 to provide a lower trigger voltage (breakdown of thecollector-base junction) for improved ESD protection. A trigger voltageof 16V, for example, may be used to provide protection of the 12V CMOSdevices having a typical junction breakdown of 20V. The NPN ESD clamp500 shown in FIG. 5 also includes DN floor isolation layer 513 andN-type sidewall isolation regions 514A and 514B to provide completeisolation of the ESD clamp 500 from P-type substrate 502. Active regionsare separated by field oxide layer 510 and contacted by metallizationlayer 512 through contact holes in ILD 511. Isolation (ISO) electrodesare connected to DN layer 513 via N+ contacts 515A and 515B. Dependingon the bias conditions of the input pad, the ISO electrodes may be tiedto the same potential as the collector 503, the same potential as thebase 501, or some other potential defined in the IC.

Clamp 500 may be an annular device, with collector 503 at the center andeach of emitters 504A, 504B, P+ base contacts 505A, 505B, and isolationregions 514A, 514B in an annular shape surrounding collector 503. Note:As used herein, the term “annular” refers to a geometrical figure havingan open center region whether the shape is circular, rectangular,hexagonal or some other shape.

FIG. 6 shows a cross-sectional view of an isolated GGNMOS ESD clamp 600.P-well region 601, which serves as the body of the NMOS 600, is isolatedfrom P-type substrate 202 by DN floor isolation layer 613 and N-typesidewall isolation regions 614A and 614B. The device also includes N+drain region 603, N+ source regions 604A and 604B, P+ body contactregions 605A and 605B, LDD regions 606, gate 607, gate oxide layer 608,sidewall spacers 609, field oxide layer 610, ILD 611, and metallizationlayer 612. The metallization contact to N+ drain 603 is separated fromthe edges of gate 607 by a significant distance, for example 1-10microns, to provide some ballasting resistance to distribute the ESDcurrent uniformly among multi-fingered GGNMOS clamp devices. P+ bodycontacts 605A and 605B are spaced from the source regions 604A and 604Bby a significant distance, for example 1-10 microns, to provide someresistance between the source regions 604A and 604B and the body 601,which lowers the bipolar snapback voltage and allows easier triggeringduring an ESD event. An optional ESD implant 616 may be includedadjacent the drain region to provide a lower trigger voltage (breakdownof the drain-body junction) for improved ESD protection. A triggervoltage of 9V, for example, may be used to provide protection of the 5VCMOS devices having a typical junction breakdown of 12V. Active regionsare separated by field oxide layer 610 and contacted by metallizationlayer 612 through contact holes in ILD 611. Isolation (ISO) electrodesare connected to DN 613 via N+contact regions 615A and 615B. Dependingon the bias conditions of the input pad, the ISO electrodes may be tiedto the same potential as the drain 603, the same potential as the body601, or some other potential defined in the IC.

Like device 500, device 600 may be annular, with gate 607, sourceregions 604A, 604B, body contacts 605A, 605B, and sidewall isolationregions 614A, 614B surrounding drain region 603.

FIG. 7A shows a cross-sectional view of an isolated ESD clamp diode 1100comprising P-type region 1103 that is isolated from P-type substrate1101 by high-energy implanted DN floor isolation layer 1102 and sidewallisolation N-wells 1105A and 1105B, which may be annular. N+ cathode 1106extends across the semiconductor surface between LOCOS oxide regions1108 and forms electrical contact with the floor isolation layer 1102through its overlap onto N-wells 1105A and 1105B. The N+ cathode (K)1106, is contacted through ILD 1109 and electrically connected by metallayer 1111 through an optional barrier metal layer 1110. P-body orP-base anode 1104 is contained within isolated P-type region 1103 andcontacted by a P+ region (not shown) within the isolated P-type region1103. The contact is preferably formed in the dimension into the page,by interrupting the N+ cathode 1106 to insert the P+ region. Contact tonon-isolated P-type substrate 1101 is facilitated by P+ regions 1107Aand 1107B, which in a preferred embodiment form a ring circumscribingthe diode 1100.

FIG. 7B shows a cross-sectional view of an isolated ESD clamp diode 1120comprising P-type region 1131 that is isolated from P-type substrate1121 by high-energy implanted DN floor isolation layer 1122 and sidewallisolation N-wells 1123A and 1123B, which may be annular. N+ cathode 1125extends across the semiconductor surface between LOCOS oxide regions1129 and forms electrical contact with the floor isolation layer 1122through its overlap onto N-wells 1123A and 1123B. The N+ cathode (K)1125, is contacted through ILD 1130 and electrically connected by metallayer 1128 through optional barrier metal layer 1127. P-well anode 1124is contained within isolated P-type region 1131 and contacted by a P+region (not shown) within the isolated P-type region 1131. The contactis preferably formed in the dimension into the page, by interrupting theN+ cathode 1125 to insert the P+ region. Contact to non-isolated P-typesubstrate 1121 is facilitated by P+ regions 1126A and 1126B, which in apreferred embodiment form a ring circumscribing the diode 1120.

Unlike a conventional diffused well which has its peak concentrationnear the surface and a monotonically decreasing concentration withincreasing depth, P-well 1124 may be formed by a high energy ionimplantation of boron, for example, and preferably by a boronchain-implant comprising a series of boron implants varying in dose andenergy. The chain implant, while it may comprise any number of implants,is graphically represented in the drawing by two regions—a surface layerPW1, and a subsurface layer PW1B, formed by ion implantation through asingle mask and without the use of epitaxy. In a preferred embodimentthe deeper layer PW1B is more highly concentrated than the surface layerPW1.

FIG. 7C shows a cross-section of an isolated ESD clamp diode 1140comprising multiple parallel N-well to P-well junctions all contained inan isolated P-type region. Isolated P-wells 1144A and 1144B arecontacted by P+ regions 1146D and 1446C, and N-wells 1143A, 1143B and1143C are contacted by N+ regions 1145A, 1145B, and 1145C. The resultingdiodes are isolated from P-type substrate 1141 by high energy implantedDN floor isolation layer 1142 and N-wells 1143A and 1143C. The device iscircumscribed by LOCOS field oxide layer 1149 and P+ substrate ring1146A and 1146B. The active areas are contacted through ILD 1150 bymetal layer 1148 through optional barrier metal layer 1147.

Unlike a conventional diffused wells which have peak concentrations nearthe surface and a monotonically decreasing concentration with increasingdepth, the P-wells 1144A and 1144B, along with N-wells 1143A, 1143B and1143C, are formed by high energy ion implantation, and preferably by achain-implant comprising a series of implants varying in dose andenergy. While the chain implants may comprise any number of implantsteps, they are graphically represented in the drawing by tworegions—surface layers PW1 and NW1, and a subsurface layers PW1B andNW1B. In a preferred embodiment the deeper layers NW1B and PW1B are morehighly concentrated than the surface layers NW1 and PW1, causing thebreakdown of the Zener diodes to occur at a location well below thesurface.

The various features shown in the isolated ESD clamp examples of FIGS.7A-7C are illustrative of structures that are compatible with thedisclosed process and capable of providing cost-effective, stacked ESDprotection devices. It is well within the scope of this invention tocombine the features from different figures to arrive at the besttermination structure for a given implementation. For example, it ispossible to add metal interconnect layers above the single metal layershown, to substitute the LOCOS field oxide layers with alternative fielddielectric schemes such as deposited and/or recessed field oxides.

FIG. 8 shows a cross-sectional view of a stacked ESD clamp structurecomprising a top ESD clamp 801 and a bottom ESD clamp 802 connected inseries. Electrical connections are illustrated schematically, showingthe ESD clamps connected between an input pad 803 and a ground pad 804.Bottom ESD clamp 802 is non-isolated (has a common terminal connected tosubstrate 805), while top ESD clamp 801 is isolated from substrate 805by DN floor, isolation layer 806 and N-type sidewall isolation regions807A and 807B.

While specific embodiments of this invention have been described, theseembodiments are illustrative only and not limiting. Persons of skill inthe art will readily see that numerous alternative embodiments arepossible in accordance with the broad principles of this invention.

1. An isolated ESD protection device, the device being connected betweenthe input terminals of a circuit to be protected, both the device andthe circuit being formed in a semiconductor substrate of a firstconductivity type, the device comprising: a floor isolation layer of asecond conductivity type; a sidewall isolation region of the secondconductivity type, the sidewall isolation region extending down from asurface of the substrate and merging with the floor isolation layer toform an isolated region of the substrate; and an ESD clamp structurewithin the isolated region wherein the ESD clamp structure comprises afield-effect transistor, the field-effect transistor comprising: asource region of the second conductivity type; a drain region of thesecond conductivity type; a gate separated from the isolated region by adielectric layer and overlying a channel region between the sourceregion and the drain region.
 2. The isolated ESD protection device ofclaim 1 further comprising a resistor connected between the gate regionand the source region, an impedance of the resistor being in a range of200 to 20,000 ohms.
 3. The isolated ESD protection device of claim 1further comprising an isolation contact region at a surface of thesubstrate within the sidewall isolation region, the isolation contactregion being in electrical contact with an isolation contact.
 4. An ESDprotection device formed in a semiconductor substrate comprising: afirst ESD clamp structure; at least one additional ESD clamp structure,the at least one additional ESD clamp structure being electricallyisolated from the substrate, all of the ESD clamp structures beingconnected in series with each other between input terminals of a circuitto be protected.
 5. The ESD protection device of claim 4 wherein each ofthe first ESD clamp structure and the at least one additional ESD clampstructure comprises a field-effect transistor.
 6. The ESD protectiondevice of claim 4 wherein the substrate is of a first conductivity typeand the at least one additional ESD clamp structure is formed within anisolated region of the substrate, the isolated region being bounded by:a floor isolation layer of a second conductivity type; a sidewallisolation region of the second conductivity type, the sidewall isolationregion extending down from a surface of the substrate and merging withthe floor isolation layer to enclose the isolated region of thesubstrate.
 7. The ESD protection device of claim 6 wherein the sidewallisolation region is annular.
 8. The ESD protection device of claim 6further comprising an isolation contact region at a surface of thesubstrate within the sidewall isolation region, the isolation contactregion being in electrical contact with an isolation contact, theisolation contact being in electrical contact with one of the inputterminals.